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Japan’s AIST develops polarisation-diverse integrated optical switch

Researchers from Japan’s National Institute of Advanced Industrial Science and Technology (AIST) have developed a new kind of silicon-photonics-based optical switch that is insensitive to optical polarisation. Their work will be presented at OFC 2017, to be held 19–23 March in Los Angeles.

Coherent optical signals in modern high-speed communications networks exploit the polarisation properties of light, by carrying a different set of data on each polarisation, a technique known as polarisation-division multiplexing. Therefore, optical switches (sometimes called optical path switches) need to manage light with both vertical and horizontal polarisations equally well.

Unfortunately, the performance of integrated optical components can be highly sensitive to polarisation. To achieve dual operation, a separate switch circuit must be used for each polarisation, which doubles the size of the chip and increases the cost of the system.

The new device, described as a “path-independent-insertion-loss (PILOSS) optical switch”, is a compact integrated device made in silicon consisting of a single 8 x 8 grid of 2 x 2 element switches. The researchers found that a single 8 x 8 grid with novel unique port assignments could take the place of two synchronised grids, and thus be used to simultaneously manage both polarisations of light.

The researchers designed the device so that the distance travelled by any signal passing through the 8 x 8 grid is identical, regardless of polarisation and which route it takes through the switch. This means that the attenuation and delay of the signals are the same, allowing for a consistently high-quality signal.

“In this way, the switch chip achieves polarization ’insensitivity’ without doubling the size and cost of the chip, which is important for broadening the practical application of such photonics integrated devices,” said lead author Ken Tanizawa of AIST. “We strongly believe that a silicon-photonic switch is a key device for achieving sustainable growth of traffic bandwidth in optical networks, including both telecommunications and data communications, and eventually computer communications.”

The new device also features polarisation splitter-rotators integrated onto the chip. The splitter-rotators take input light signals with both polarizations, divide them into separate horizontal and vertical polarizations, and rotate one 90 degrees to match the orientation of the other. Both polarisations are synchronously switched on the single 8 x 8 grid with the unique port assignments. The switched polarisations are then recombined by the polarisation splitter-rotator so that the original signals are recovered.

The new switch is a proof-of-concept design, and the AIST team is now working to improve the device and to create a design with a larger number of ports. As well as enhancing network flexibility, such tiny, polarisation-insensitive switches could open up new possibilities for the use of optical switching in future energy-efficient optical networks, the researchers say.

Semiconductor foundry CST Global is leading a UK government-funded project that could substantially reduce the cost of manufacturing high-speed laser diodes for next-generation fibre-to-the-premises (FTTP) networks.

The market for passive optical networks (PON) is expanding rapidly as consumers upgrade their broadband connections to higher speeds. Indeed, this summer, CST Global said it had shipped more than 25 million lasers into PON markets worldwide. To meet the insatiable demand for bandwidth, the next generation of PON lasers will need to be both higher speed and lower cost.